In a known common boat propeller drive unit of this type, a so-called inboard-outboard drive, in which the exhaust is conducted through a duct in the anti-cavitation plate, the distal end of the anti-cavitation plate is open aft and forms a rearwardly directed exhaust ejector. When operating planing motorboats equipped with such drive units at planing speed,, the exhaust is ejected directly out into the atmosphere, which has the advantage of low exhaust counter-pressure. In boats with vertical transoms with a relatively large surface and large superstructures, back-suction can occur however at certain speeds, with the result: that a cloud of exhaust will remain behind the transom and accompany the boat. In boats with the cockpit aft, exhaust can even be sucked into the cockpit creating discomfort to the passengers. Another common method of conducting exhaust away from a motorboat with an inboard-outboard drive unit is to allow the exhaust to exit through an exhaust duct in the propeller hub. The exhaust is thus ejected into the water even at planing speeds and the risk of exhaust being sucked into the cockpit due to back-suction is eliminated. Exhaust ejection through the propeller hub below the surface of the water creates however a higher exhaust counter-pressure than ejection through the cavitation plate directly out into the atmosphere.